Currently, neural stimulation has been widely applied in neural prosthesis. For example, the cochlear implant allows a patient to perceive sounds of different frequencies by generating electronic signals to stimulate the patient's auditory nerves; and the artificial retina allows a patient to have visual perception by generating electric stimulation to the patient's retina or visual cortex. Since the number of electrodes that can be provided with the neural prosthesis is limited and much less than the quantity of the human nerves, there is a relatively large difference between the sound perceived through electrical stimulation signals and the sound perceived through normal hearing persons. For the patients to have better and improved perception, it is very important to establish a proper stimulation strategy, so that the stimulation signals can generate a perception that is closely match the perception generated by the original sound signals.
Taking the cochlear implant as an example, when the electrodes have been implanted in the patient's cochlea, each of the implanted electrodes corresponds to a center frequency. That is, when one of the electrodes is turned on, the patient would perceive a sound signal having a specific frequency, and this process is referred to as forming a fixed channel. Currently, there are three most common electrical stimulation strategies. In the first electrical stimulation strategy, for example the ACE strategy proposed by Cochlear, an Australian company, the sound signal is decomposed to twenty-two frequency bands, and twenty-two electrodes are provided for simulating the twenty-two frequency bands respectively. In other words, each of the frequency bands corresponds to one electrode, and each of the electrodes is used to provide a fixed channel with a specific frequency. Then, 8-16 frequency bands having the higher energy values are selected, and the 8-16 corresponding electrodes are turning on in one single cycle. The electric current is input to these electrodes to generate electrode stimulation signals. In the second electrical stimulation strategy, the sound signal is also decomposed to several frequency bands, and each of the bands corresponds to one electrode. An energy value is extracted from each of the bands, and the electrodes separately corresponding to different bands are sequentially turned on in different cycles. That is, the electrodes are sequentially turned on in continuous cycles to present signals of all bands.
In the above two electrical stimulation strategies, fixed channels are used to synthesize the original sound signal, which means the simulated sound signals perceived by the patient are synthesized by fixed frequency components. However, as shown in FIG. 1, to more accurately reproduce the original sound signal spectrum, it is necessary to select the frequencies with the highest energy values for synthesizing the sound signal. That is, in the use of fixed channels to synthesize the original sound signal spectrum, it is possible the frequencies with the highest energy values in the original sound signal fall out of the range of the center frequencies of the electrodes. As a result, there would be a relatively large difference between the stimulating signal spectrum and the original sound signal spectrum.
In the third electrical stimulation strategy, a stimulation signal having a frequency ranged between the center frequencies of two adjacent electrodes is generated by turning on the two adjacent electrodes at the same time through current steering, and this process is referred to as forming a virtual channel. With current steering, the frequencies with higher energy values in the original sound signal can be generated. With the third electrical stimulation strategy, the stimulation sound signal spectrum and its peaks resemble the original sound signal spectrum better, as can be seen in FIG. 1. According to the third electrical stimulation strategy, the sound signal spectrum is first divided into several frequency bands, and the energy value of each of the bands is obtained in order to locate the frequencies corresponding to the highest energy values in all bands. Then, in each of several continuous cycles, two adjacent electrodes are simultaneously turned on, so as to create a frequency corresponding to the highest energy value in the band corresponding to the two adjacent electrodes and generate a stimulation signal for the band. By sequentially turning on different pairs of two adjacent electrodes in continuous cycles to obtain stimulation signals for all bands, stimulation sound signal spectrums which closely approximate the original sound signal spectrums can be generated.
Please refer to FIG. 2 that shows the third electrical stimulation strategy according to prior art. As shown, according to the prior art third electrical stimulation strategy, a sound signal received by a microphone is first divided into a plurality of frequency bands, such as 15 bands. Then, the highest energy value, or a spectral peak, of each of the bands and the frequency corresponding thereto are obtained. That is, 15 frequencies and 15 spectral peaks thereof that are to be generated. And, total 15 groups of two adjacent electrodes separately corresponding to the 15 bands are turned on in a fixed sequence in 15 continuous cycles. In FIG. 2, the groups of two adjacent electrodes are turned on in the sequence of 8, 11, 12, 9, 2, 13 . . . , 6, 7 and 15 in fifteen cycles. The amount of electric current supplied to each of the electrodes is decided according to the amplitude of the spectral peak of the band, so as to generate the sound signal as received by the microphone.
The neural stimulation techniques using the above three electrical stimulation strategies still have the problems of poor electrode utilization efficiency and large discrepancy between the original sound signal spectrum and the stimulation sound signal spectrum as perceived by the patient.